EP3968064B1 - Optische filtervorrichtung - Google Patents

Optische filtervorrichtung Download PDF

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Publication number
EP3968064B1
EP3968064B1 EP21191574.9A EP21191574A EP3968064B1 EP 3968064 B1 EP3968064 B1 EP 3968064B1 EP 21191574 A EP21191574 A EP 21191574A EP 3968064 B1 EP3968064 B1 EP 3968064B1
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EP
European Patent Office
Prior art keywords
slots
wavelength
reflective elements
filter
refractive index
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EP21191574.9A
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English (en)
French (fr)
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EP3968064A1 (de
Inventor
Jérôme Le Perchec
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Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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Commissariat a lEnergie Atomique CEA
Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
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    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/26—Reflecting filters
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1809—Diffraction gratings with pitch less than or comparable to the wavelength
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1814—Diffraction gratings structurally combined with one or more further optical elements, e.g. lenses, mirrors, prisms or other diffraction gratings
    • G02B5/1819—Plural gratings positioned on the same surface, e.g. array of gratings
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1847—Manufacturing methods
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/18—Diffraction gratings
    • G02B5/1866—Transmission gratings characterised by their structure, e.g. step profile, contours of substrate or grooves, pitch variations, materials
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/204—Filters in which spectral selection is performed by means of a conductive grid or array, e.g. frequency selective surfaces
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/0121—Operation of devices; Circuit arrangements, not otherwise provided for in this subclass
    • G—PHYSICS
    • G02—OPTICS
    • G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
    • G02B5/00—Optical elements other than lenses
    • G02B5/20—Filters
    • G02B5/208—Filters for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2203/00—Function characteristic
    • G02F2203/05—Function characteristic wavelength dependent
    • G02F2203/055—Function characteristic wavelength dependent wavelength filtering

Definitions

  • the present invention relates to an optical filtering device having a transmission response with a maximum at a wavelength ⁇ 0 .
  • the invention also relates to an adjustable optical filtering apparatus incorporating this optical filtering device.
  • Such filters find particular application in the field of visible and infrared photo-detection, more particularly intended for the spectral detection of gases, and for visible and infrared color imaging.
  • Micro- or nano-structured optical filtering devices are generally preferred for their high compactness which allows them to be integrated into photodetectors.
  • the filtering device has a narrow transmission bandwidth around the wavelength ⁇ 0 and a significant rejection rate outside this bandwidth. This narrow bandwidth is also called "transmission peak" in this text.
  • the transmission rate is equal to the ratio between the quantity of electromagnetic radiation incident on the optical filtering device and the quantity of electromagnetic radiation having completely passed through the device.
  • the transmission rate is equal to 1 or 100%, at a wavelength ⁇ , when the device is completely transparent at this wavelength ⁇ .
  • the rejection rate is the inverse of the transmission rate.
  • the rejection rate is equal to 1 or 100%, at a wavelength ⁇ , when the device is completely opaque to electromagnetic radiation at this wavelength ⁇ .
  • This known optical filtering device comprises reflector elements deposited on one face of a half-wave plate. These reflective elements define a periodic sub-wavelength array of parallel through-slits.
  • the assembly formed by this network and the waveguide plate constitutes a resonator having the behavior of a band-pass filter around the wavelength ⁇ 0 .
  • the invention aims to propose an optical filtering device which makes it possible to increase the transmission rate without degrading, or even improving, the rejection rate and selectivity.
  • the subject of the invention is an optical filtering device according to claim 1.
  • the invention also relates to an adjustable optical filtering device and a detector implementing the above optical filtering device.
  • FIG. 1 represents an optical filtering device 2.
  • the transmission response of device 2 has a single transmission peak corresponding to a high transmission rate.
  • a transmission rate is said to be "high” if it exceeds 80% or 85%.
  • This high transmission rate is obtained for a wavelength ⁇ 0 .
  • the device 2 On either side of the transmission peak, the device 2 has a high rejection rate, that is to say a rejection rate greater than 95% or 99%, or even greater than 99.9%.
  • This high rejection rate is observed over wavelength ranges located to the right and left of the transmission peak. These ranges each extend over a width greater than ⁇ 0 /2.
  • the transmission peak of device 2 is in the mid-infrared, that is to say between a wavelength of 3 ⁇ m and a wavelength of 5 ⁇ m.
  • the wavelength ⁇ 0 is equal to 4 ⁇ m.
  • the device 2 comprises two monodirectional optical filters 4 and 6 arranged one above the other in a vertical direction and separated from each other by an intermediate layer 8.
  • the orientation in space is located in relation to an orthogonal reference XYZ.
  • the Z direction is the vertical direction.
  • the X and Y directions are contained in a horizontal plane.
  • the direction are defined in relation to the Z direction.
  • the filter 4 is located below the filter 6 and produced on the upper face of a dielectric medium 10.
  • the medium 10 has a refractive index n 10 at the wavelength ⁇ 0 and a thickness h 10 in the direction vertical.
  • the medium 10 is a solid non-metallic substrate.
  • the direction of the incident electromagnetic radiation is represented by an arrow I on the figure 1 .
  • Incident electromagnetic radiation propagates vertically from top to bottom.
  • the incident electromagnetic radiation has a TM ("Transverse Magnetic") polarization with the electric field in the XZ plane.
  • Filter 4 has a very high transmission peak around the length ⁇ 0 .
  • the maximum transmission rate of filter 4 is greater than 90%.
  • the filter 4 is designed in accordance with the teaching of the patent US8937277B2 except that the width w of the slots is not necessarily chosen less than P/3. In this example of embodiment, the width w is chosen greater than P/3 but less than 2P/3.
  • Filter 4 extends mainly horizontally. In the vertical direction, it extends from a lower bordering plane P 4inf to an upper bordering plane P 4sup .
  • the P 4inf and P 4sup planes are horizontal.
  • the planes P 4inf and P 4sup are located at the interface between the filter 4 and external media of low refractive indices located, respectively, below the plane P 4inf and above the plane P 4sup .
  • the medium of low refractive index located under the plane P 4inf is medium 10.
  • the medium of low refractive index located above the plane P 4sup is layer 8.
  • the filter 6 extends from a lower border plane P 6inf to an upper border plane P 6sup .
  • the planes P 6inf and P 6sup are here the symmetrical ones, respectively, of the planes P 4sup and P 4inf with respect to a median horizontal plane P m .
  • the plane P m is the median plane of layer 8. It is located halfway up layer 8.
  • the planes P 6inf and P 6sup are located at the interface between the filter 6 and external media of low refractive indices.
  • the low refractive index medium located under the plane P 6inf is layer 8.
  • the low refractive index medium located above the plane P 6sup is a medium 20.
  • the medium 20 is a gaseous medium whose index n 20 of refraction at the wavelength ⁇ 0 is less than the index n 22 and, preferably, less than 3n 22 /5 or less to 1.5.
  • medium 20 is air. Air is particularly advantageous because it has a refractive index close to 1.
  • Blade 22 is identical to blade 12 and performs the same function.
  • the blade 22 is symmetrical to the blade 12 with respect to the plane P m .
  • the reflector elements 24 are identical to the reflector elements 14. Furthermore, they are arranged relative to each other in the same manner as the reflector elements 14. In this embodiment, the reflector elements 24 are symmetrical to the reflector elements 14 with respect to the plane P m .
  • Layer 8 is made of a dielectric material having a refractive index n 8 at the wavelength ⁇ 0 .
  • the refractive index n 8 is low, that is to say less than 3n 12 /5 and, preferably, less than 1.5.
  • the thickness of layer 8 is denoted h 8 .
  • Layer 8 extends from plane P 4sup to plane P 6inf .
  • the thickness h 8 is therefore equal to the distance which separates the planes P 4sup and P 6inf .
  • This thickness h 8 is chosen to create a moderate coupling between the evanescent fields generated by filters 4 and 6. Such a choice of thickness h 8 is explained in detail later.
  • the plate 12 is transparent at the wavelength ⁇ 0 and forms a waveguide and, preferably, a half-wave plate.
  • the blade 12 is made of a dielectric material with a high refractive index.
  • an element made of a material X means that this material X represents at least 90% or 95% of the mass of the element index n 12 is greater than indices n 10 and n 8 and, preferably, greater than 5n 10 /3 and 5n 8 /3.
  • the thickness h 12 of the plate 12 is chosen to form a half-wave plate in a range of wavelengths including the wavelength ⁇ 0 and thus obtain an electromagnetic resonance in the plate 12.
  • the thickness h 12 is between 0.8 ⁇ 0 /(2n 12 ) and 1.2 ⁇ 0 /(2n 12 ).
  • the thickness h 12 is chosen slightly less than ⁇ 0 /(2n 12 ). This makes it possible to make the spectral response better than if the strict law ⁇ 0 /(2n 12 ) of the guided mode alone is respected. An electromagnetic resonance is thus obtained in the blade 12.
  • the reflecting elements 14 are made of a material whose electrical permittivity is negative at the wavelength ⁇ 0 .
  • the reflective elements 14 are made of metal and, in this example, of aluminum.
  • the reflector elements 14 define a periodic network of slots 30 intended to be traversed by the incident electromagnetic radiation.
  • the number of slots 30 is greater than ⁇ 0 / ⁇ , where ⁇ is the width at half height of the transmission peak at the wavelength ⁇ 0 .
  • the network of slots 30 is formed on the upper face of the blade 12.
  • the slots 30 open onto the upper face of the blade 12. The characteristics of the slots 30 are chosen so that the network has a selectivity in transmission around the wavelength ⁇ 0 .
  • the thickness h of the reflector elements 14 in the vertical direction is low, that is to say less than 50 nm or 100 nm.
  • the thickness h met is also greater than the skin thickness p met ( ⁇ 0 ), at the wavelength ⁇ 0 , of the material used to make the reflective elements 14.
  • the thickness h met is between p met ( ⁇ 0 ) and 5p met ( ⁇ 0 ).
  • the elements 14 are strips which extend continuously in the Y direction and which are deposited on the blade 12.
  • the elements 14 form a single periodic network of slots extending continuously in the Y direction, of period P.
  • the optical filtering property of interest is that obtained when the polarization of the incident electromagnetic radiation I is a rectilinear polarization called transverse magnetic, that is to say with an electric field in the XZ plane and a magnetic field parallel to the Y direction.
  • the period P is equal to the sum of the width L of an element 14 and the width w of a slot 30.
  • the period P is chosen to satisfy the following condition: ⁇ 0 /n 12 ⁇ P ⁇ 2 ⁇ 0 / No. 12 .
  • the rejection rates to the right and left of the transmission peak are quite bad. For example, they are less than 90% over a spectral range [0.75 ⁇ 0 ; 1.25 ⁇ 0 ].
  • the reflective elements 14 are arranged in a medium with a refractive index less than 1.5.
  • this middle is layer 8.
  • the thickness h 8 is chosen between 1.25 ⁇ and 2.75 ⁇ or between 1.5 ⁇ and 2.5 ⁇ or between 1.9 ⁇ and 2.1 ⁇ . In this embodiment, the thickness h 8 is equal to 2 ⁇ . The choice of this thickness h 8 is justified by the experimental results below.
  • the coefficient ⁇ is equal to 277 nm.
  • the thickness h 8 must be chosen between 346 nm and 761 nm.
  • the thickness h 8 is equal to 550 nm.
  • the maximum amplitude of the transmission peak of isolated filter 4 is very high and reaches 94% for the wavelength equal to 4.07 ⁇ m.
  • the maximum amplitude of the transmission rate of device 2 is also very high since it reaches 92% for the wavelength equal to 3.98 ⁇ m.
  • the maximum amplitude of the transmission peak of device 2 is better than that which would be obtained in the absence of moderate coupling of the evanescent fields of filters 4 and 6.
  • the right and left rejection rate of the isolated filter 4 transmission peak is less than 90%. Unlike isolated filter 4, the rejection rate to the right and left of the transmission peak of device 2 is very high and remains greater than 99%. For example, the rejection rate is equal to 87.4% at the wavelength equal to 3.5 ⁇ m for isolated filter 4 while it is equal to 99.4% for device 2.
  • device 2 has both a very high transmission rate and a very high rejection rate outside the transmission peak.
  • the resonance wavelength of device 2 is also slightly offset from that of isolated filter 4.
  • the thickness h 8 is chosen less than ⁇ , then the coupling of the evanescent fields of filters 4 and 6 in layer 8 is strong and no longer moderate. It was also determined that strong coupling completely distorts the transmission peak and degrades the transmission rate and selectivity of device 2. This is illustrated in Figure 2. figure 4 .
  • the graph of the figure 4 represents the transmission responses of the device 2 obtained by simulation for different thicknesses h 8 . For these simulations, the other numerical values which characterize the device 2 are equal to those used to obtain the graph of the Figure 3 . More precisely, the curves 60, 62, 64, 66 and 68 were obtained for values of the thickness h 8 equal, respectively, to 50 nm, 200 nm, 400 nm, 600 nm and 1200 nm.
  • Curves 60 and 62 correspond to strong coupling of the evanescent fields. It can be observed that such strong coupling splits the transmission peak into at least two, which degrades the selectivity of device 2 as well as the rejection rate outside the transmission peak.
  • the thickness of layers 74 and 76 is equal to the thickness h met .
  • the upper face of layer 76 is directly exposed to the outside.
  • the medium 10 and the layer 8 are made of amorphous silicon oxide whose refractive index is approximately 1.4 for a wavelength ⁇ 0 equal to 4 ⁇ m.
  • the blade 12 is made of amorphous silicon whose refractive index is approximately 3.84 for the wavelength ⁇ 0 .
  • the stack 72 is etched to produce the reflective elements 14 and 24 in the layers, respectively, 74 and 76.
  • trenches 82 ( Figure 7 ) vertical are dug from the exterior face of layer 76 and stop on the upper face of blade 12. Each of these trenches 82 successively passes through layer 76, layer 8 and layer 74.
  • the trenches 82 are filled with a dielectric material 88 whose refractive index is low ( Figure 8 ).
  • material 88 is silicon oxide.
  • the upper face is polished, for example by a mechanical-chemical polishing process, to make it flat and expose the upper face of the reflective elements 24.
  • the blade 22 is deposited on the exposed face of the reflective elements 24 previously produced ( Figure 9 ).
  • a coating 94 ( Figure 9 ) is deposited on the upper face of the blade 24.
  • the coating 94 is a protective layer or an anti-reflective layer.
  • This process begins by providing, during a step 100, a stack identical to stack 72 except that layers 8 and 76 are omitted.
  • the metallization layer 74 is etched to form the reflective elements 14 on the upper face of the blade 12 ( Figure 11 ).
  • the intermediate layer 8 is deposited on the reflective elements 14 ( Figure 12 ). Since the reflective elements 14 are separated from each other by slots, the face on which the layer 8 is deposited is not flat. As a result, the upper face of layer 8 after its deposition is not flat either. It has roughness facing each reflective element 14.
  • the reflective elements 24 are produced on the upper face of the layer 8.
  • the upper face of the layer 8 is not polished before producing the reflective elements 24.
  • the reflective elements 14 are produced by depositing a metallization layer on the upper face of the layer 8 then by etching this metallization layer.
  • the blade 22 is deposited on the reflective elements 24 ( Figure 14 ).
  • the blade 22 is deposited on a face which is not flat, its upper face has asperities facing each of the reflective elements 24.
  • the blade 22 is not exactly symmetrical to the blade 12 with respect to the plane P m .
  • the optical filtering device obtained has a high transmission rate and high rejection rates on either side of the transmission peak.
  • FIG. 15 represents an optical filtering device 120 identical to device 2 except that the filter 6 is replaced by an optical filter 122.
  • the filter 122 is identical to the filter 6 except that the order in which the blade 22 and the reflective elements 24 are stacked l 'one on top of the other is reversed.
  • the lower face of the blade 22 is flush with the plane P 6inf and the reflective elements 24 are arranged on the upper face of the blade 22 in contact with the medium 20.
  • FIG 16 represents the transmission response of device 2 (curve 130) and device 120 (curve 132).
  • the scale of the y-axis is logarithmic. It can be observed that the maximum amplitude of the transmission peak of device 2 is slightly higher than the maximum amplitude of the peak transmission of the device 120. Here, a difference of approximately 1.6% is observed.
  • the device 2 has a rejection rate slightly higher than that of the device 120. However, although the performance of the device 120 is a little lower than that of the device 2, the transmission and rejection rates obtained with the device 120 remain much better than those obtained with isolated filter 4 or with a superposition of filters 4 and 6 but without moderate coupling of their evanescent fields.
  • the graph of the Figure 16 also includes a curve 134 which corresponds to an optical filtering device identical to device 2 except that the reflecting elements 24 are offset, for example in the direction X, by a pitch equal to P/4.
  • the performances of this embodiment are a little lower than those of the device 2 but the transmission and rejection rates obtained remain much better than those obtained with the isolated filter 4 or with the superposition of filters 4 and 6 but without moderate coupling of their evanescent fields.
  • FIG 17 represents an optical filtering device 140.
  • the device 140 is identical to the device 120 except that the medium 10 is deposited on a support 142 which has a high refractive index n 142 .
  • the index n 142 is greater than 1.5 or 2.
  • the support 142 is made of silicon or germanium.
  • the medium 10 preferably forms a quarter-wave plate.
  • the thickness h 10 of the medium 10 is chosen between 0.9M ⁇ 0 /(4n 10 ) and 1.1M ⁇ 0 /(4n 10 ), where M is an odd integer chosen in advance.
  • the thickness h 10 is equal to M ⁇ 0 /(4n 10 ).
  • the apparatus 150 includes an optical filtering device 152 and a voltage source 154.
  • the device 152 is, for example, identical to the device 2 except that it has two connection terminals 156 and 158. All the reflecting elements 14 are electrically connected to the terminal 156. All the reflecting elements 24 are electrically connected to the terminal 158 The reflector elements 14 and the terminal 156 are electrically isolated from the reflector elements 24 and the terminal 158.
  • the reflector elements 14 are electrically connected to each other via very fine wires 160.
  • the width of these wires 160 is less than 150 nm or 100 nm.
  • the width of the wires 160 is equal to 100 nm.
  • At least one of these wires 160 electrically connects the elements 14 to terminal 156.
  • the reflecting elements 24 are connected to the terminal 158 in a manner similar to what has been described for the reflecting elements 14. Thus, when a potential difference is generated between the terminals 156 and 158, an electric field is generated in the layer intermediate 8.
  • the source 154 is electrically connected between terminals 156 and 158. It is capable of generating a difference in potentials between these terminals 156 and 158.
  • the source 154 comprises a module 170 for adjusting the difference in potentials between terminals 156 and 158. 158.
  • the potential difference between terminals 156 and 158 using module 170, it is possible to modify certain characteristics of the device 152 such as, for example, the position of its transmission peak or the modulation of its amplitude. For example, with height h 8 equal to 500 nm, a potential difference U equal to 50 Vdc and a coefficient R equal to 100 pm/V, the variation ⁇ n 8 of the index n 8 of refraction is equal to 0.1.
  • FIG. 19 represents a 180 multi-pixel detector.
  • This detector 180 comprises a matrix of cells for detecting incident electromagnetic radiation.
  • each of these cells is formed by a detector diode made in a semiconductor substrate 186.
  • a detector diode made in a semiconductor substrate 186.
  • FIG. 19 only two detector diodes 182 and 184 are shown. The junctions of these diodes 182, 184 are made in this substrate 186.
  • optical filtering device is placed in front of each of these diodes.
  • this filtering device is, for example, identical to the device 140 except that the support 142 is replaced by the substrate 186.
  • the optical filtering devices located in front of the diodes 182, 184 bear the numerical references, respectively, 140a and 140b.
  • the substrate 186 and the medium 10 is common to all optical filtering devices.
  • the substrate 186 and the medium 10 therefore extend horizontally and continuously in front of each of the detector diodes.
  • a trench 190 is dug along the periphery of each of the devices 140a and 140b in order to minimize interference between these different optical filtering devices.
  • the trench 190 separates the blades 12, 22 and the intermediate layer 8 from each of the filtering devices of the blades 12, 22 and the intermediate layer 8 from the filtering devices located immediately next to it.
  • the characteristics of the device 140a may be different from the characteristics of the device 140b.
  • the period P of the device 140a is different from the period P of the device 140b.
  • FIG. 20 represents another multi-pixel detector 190 identical to the detector 180 except that each detection cell is formed by a micrometer-sized bolometer.
  • each detection cell is formed by a micrometer-sized bolometer.
  • the bolometer 192 comprises a thermo-resistive membrane 196 supported by pillars 200, 202, above a reflective substrate 204.
  • the distance which separates the substrate 204 from the membrane 196 is equal to or approximately equal to a quarter of the wavelength at absorb.
  • each bolometer is encapsulated in a hollow cavity to isolate it from the external environment, as explained in particular in the patent EP1243903B1 .
  • the walls of this cavity are, for example, made of amorphous silicon.
  • a filtering device is made above each bolometer.
  • the filtering devices located above, respectively, the bolometers 192 and 194 bear the numerical references, respectively, 208 and 210.
  • the filtering devices 208 and 210 are identical, for example, to the device 2.
  • the medium 10 of each of these filtering devices corresponds to the medium filling the interior of the hollowed-out cavity inside which the bolometer is located.
  • the blade 12 forms the upper wall of the hollowed-out cavity.
  • the thickness h 10 of the medium 10 is equal to the distance separating the blade 12 from the membrane 196 of the bolometer. As described above, preferably, the thickness h 10 is between 0.9M ⁇ 0 /(4n 10 ) and 1.1M ⁇ 0 /(4n 10 ), where M is an odd integer chosen in advance. Typically, in the case of a bolometer, the medium 10 is vacuum.
  • FIG. 21 represents another possible embodiment of a two-dimensional filter 230 capable of being used in place of filters 4 and 6.
  • Filter 230 is identical to filter 4 except that the reflecting elements 14 are replaced by reflecting elements 232.
  • the reflector elements 232 are identical to the reflector elements 14 except that they each have a rectangular horizontal section and therefore form pads aligned one behind the other in the directions X and Y.
  • the reflector elements 232 are arranged on relative to each other so as to define two periodic networks of slots 234 and 236 of respective widths w 234 and w 236 and respective periods P 234 and P 236 . These two networks are orthogonal to each other. In this case, by taking the periods P 234 and P 236 equal and by taking the widths w 234 and w 236 equal, the filter 230 is insensitive to the polarization of the incident electromagnetic radiation.
  • the rejection rate on either side of the transmission peak is improved .
  • the widths w 234 and w 236 are chosen relatively narrow, namely between P 234 /5 to P 234 /3.
  • the thickness of the reflective elements 232 is preferably close to 50 nm and less than 100 nm. Indeed, when two-dimensional filters are used instead of filters monodimensional, parasitic secondary peaks, linked to another transverse waveguide mode in the blades 12 and 22, can appear on either side of the main transmission peak, and then alter the rejection rate. However, the excitation of these secondary peaks is favored with too great a width w 234 .
  • the width w 234 must be adapted to obtain an acceptable compromise between the desired rejection rate over the entire spectral range of study and the amplitude of the desired transmission peak.
  • an optical filtering device made with two-dimensional filters makes it possible to obtain better rejection rates than those obtained with filter 230 alone.
  • the reflecting elements 24 are not necessarily symmetrical to the reflecting elements 14. For example, they are offset in any horizontal direction by a distance less than the period P.
  • the reflecting elements 24 are offset, in a horizontal direction, by a distance equal to P/2 or P/4.
  • the elements 24 are arranged in relation to each other like the elements 14 but are no longer symmetrical to the elements 14 in relation to the plane P m .
  • the reflective elements are deposited on each of the faces of the blade 12.
  • the blade 12 comprises a first network of slots on its lower face and a second network of slots on its upper face.
  • these first and second networks of slots are structurally identical.
  • they are each identical to the network of slots defined by the reflective elements 14 or 232.
  • reflective elements are deposited on each of the faces of the blade 22 and the networks of slots formed on each of the faces of the blade 22 are identical, respectively, to the first and second arrays of slots.
  • the reflective elements can be made of metals other than aluminum.
  • they are made of silver.
  • Aluminum and silver are good choices, in particular, for the production of an optical filtering device whose transmission peak is in the visible range, that is to say typically between 400 nm and 800 nm.
  • other metals can also be used such as gold, copper, platinum or an alloy thereof.
  • metals can be replaced by any material having reflective properties similar to those of metals in the wavelength range targeted by the filtering.
  • a material having a relative permittivity with a high imaginary part namely of the order of 10 or more, that is to say a highly conductive or strongly absorbing material, can be used.
  • silicon can be used in the ultraviolet (10 nm to 380 nm) and heavily N-doped silicon can be used in the infrared.
  • An ionic crystal having a negative permittivity can also be used, such as SiC in the far infrared (around 10-12 ⁇ m) for example.
  • the reflective elements forming the networks of slots are deposited directly on the blades 12 and 22.
  • a thin layer is provided, of the order of a tenth of the thickness of the blade , and of refractive index lower than that of the blade, so as to modify the refractive index in contact with the blade.
  • This additional thin layer is for example a gradient index layer and can also fulfill another function, such as an adhesion, passivation or protection function.
  • the width w can also be chosen less than P/3 to have a very selective filter having a better rejection rate than that obtained with a filter constructed in accordance with teaching on demand US8937277B2 . In this case, the transmission rate is not necessarily improved.
  • the blades 12 and 22 can be made of GaP, AlAs, GaSb, or PbTe which have high refractive indices in the optical domain.
  • GaP is a good choice if the device's transmission peak is in the visible range.
  • a silicon nitride Si x N 1-x rich in silicon (x>1), less expensive, can be used in the visible.
  • the material used can be one of the following materials: MgF 2 , BaF 2 , CaF 2 , LiF.
  • MgF 2 is a good choice if the device's transmission peak is in the visible region.
  • Layer 8 and media 10 and 20 can also be made by stacking several materials with low refractive indices.
  • Layer 8 and medium 10 are not necessarily made of a material in the solid state. They can also be made using a material in the gaseous state, such as air, or liquid. In the case of a material in the gaseous or liquid state, shims made of a material in the solid state are interposed between the filters 4 and 6 to obtain the desired thickness of the intermediate layer 8.
  • a material in the gaseous state for producing layer 8 is advantageous in particular if the transmission peak is located in the range of visible wavelengths.
  • the intermediate layer 8 is a polarizable transparent conductive oxide (for example ITO), electrically connected to a third terminal, and the reflecting elements 14 and 24 are located, respectively, in the planes P 4inf and P 6sup of the stack and each connected to terminals 156 and 158.
  • ITO transparent conductive oxide
  • an electric field between the intermediate layer 8 and the reflective elements can be applied to modify the refractive index of the waveguide blades 12 and 22.
  • the medium 20 is not necessarily a gaseous medium.
  • the medium 20 can also be a material in the solid or liquid state whose refractive index satisfies the same conditions as those stated for the medium 20.
  • Layer 8 and media 10 and 20 are non-metallic.
  • the reflector elements 14 are electrically connected to the terminal 156 via an electrically conductive film transparent to incident electromagnetic radiation.
  • This film is for example deposited over the entire upper surface of the reflective elements 14 and the slots 30.
  • this film can be made of ITO ("Indium Tin Oxide) and have a thickness of approximately 50 nm.
  • the electric field in layer 8 is generated using conductive plates located, respectively, above the blade 22 and below the blade 12 and no longer at the using the reflective elements 14 and 24. In this case, it is these conductive plates which are connected to terminals 156, 158.
  • Layer 8 of device 152 can be made of other electro-optical materials.
  • layer 8 can be made from nematic liquid crystals, whose effective refractive index, of the order of 1.5 to 2, can be modified by application of a polarization voltage applied between the terminals. 156 and 158.
  • the modification of the diffraction index n 8 is caused by a change in orientation of the molecules of layer 8. This was demonstrated, for example, in the article “ Polarization-independent actively tunable color generation on imprinted plasmonic surfaces” by D. Franklin et al., Nat Commun 6, 7337 (2015 ).
  • the optical filtering device comprises more than two identical optical filters stacked one above the other in the Z direction.
  • the intermediate layer which separates two immediately consecutive optical filters in the Z direction is produced as described for layer 8.
  • optical filters described in the patent US8937277 can be applied to the case of optical filters of the filtering devices described here.
  • the device 2 described above with a network of bands extended in the Y direction does not present a transmission peak whose amplitude is significant for polarized incident electromagnetic radiation TE (“electric transverse”) whose electric field is parallel to the direction Y.
  • TE polarized incident electromagnetic radiation
  • the article “ Efficient and monolithic polarization conversion system based on a polarization grating”, by J. Kim et al, Applied Optics 51, 4852 (2012 )) describes such a microstructured and compact system with a conversion efficiency close to 90%.
  • the electromagnetic radiation whose polarization has been converted then passes through device 2.
  • the combination of the conversion system and device 2 makes it possible to maximize the power of the flow transmitted by the filtering device.
  • device 2 has a transmission peak higher than the square of the amplitude of the peaks of filters 4 and 6 without necessarily being greater than 85% or 90%.
  • rejection rate of device 2 is not necessarily greater than 90% but remains in all cases higher than that of filters 4 and 6 taken separately.
  • each of the optical filters conforms to practically all the teachings of the patent US8937277B1 makes it possible to obtain an optical filtering device which retains the advantages of the optical filters described in this patent.
  • the optical filtering device is compact. It is adjustable in a wide range of wavelengths, ranging from 250 nm to around a hundred micrometers. It has a selective transmission bandwidth, namely a width at half maximum of the transmission peak less than 10% of the wavelength for which the peak is maximum.
  • the device presents both a higher transmission rate and a higher rejection rate over a spectral range around the peak of transmission with a width at least equal to half the wavelength for which the peak is maximum.
  • the rejection rate is typically greater than 90%.
  • Making the reflective elements in silver or aluminum makes it possible to minimize optical losses by absorption and to improve the transmission rate, particularly when the wavelength ⁇ 0 is in the visible range.
  • the transmission rate is improved.
  • the fact of making the intermediate layer in a material which is also electro-optical makes it possible to dynamically modify the characteristics of the optical filtering device.
  • the reflecting elements are rectangular makes it possible to obtain a network of slots parallel to the direction parallel to the slits and, also, the orthogonally polarized component.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Optics & Photonics (AREA)
  • Nonlinear Science (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • Engineering & Computer Science (AREA)
  • Manufacturing & Machinery (AREA)
  • Optical Filters (AREA)
  • Light Guides In General And Applications Therefor (AREA)
  • Mechanical Light Control Or Optical Switches (AREA)

Claims (15)

  1. Optische Filtervorrichtung, die eine Transmissionsantwort mit einem Maximum bei einer Wellenlänge λ0 aufweist, wobei diese Vorrichtung umfasst:
    - einen ersten optischen Filter (4; 230), der sich von einer ersten unteren Grenzebene (P4inf) aus bis zu einer parallelen ersten oberen Grenzebene (P4sup) erstreckt, wobei jede dieser ersten Grenzebenen an der Grenzfläche zwischen dem ersten Filter und einem Außenmilieu gelegen ist, wobei dieser erste optische Filter aufeinander folgend von der ersten Grenzebene zu der anderen ersten Grenzebene hin umfasst:
    - eine Scheibe (12), die einen Brechungsindex n12 bei der Wellenlänge λ0 aufweist und deren Dicke zwischen 0,8λ0/(2n12) und 1,2λ0/(2n12) beträgt, wobei der Brechungsindex n12 größer als 5n10/3 ist, wobei n10 der größte der Brechungsindizes ist, der aus der Gruppe gewählt ist, die aus den Brechungsindizes bei der Wellenlänge λ0 der an der Grenzfläche zu dem ersten Filter gelegenen Milieus besteht,
    - reflektierende Elemente (14; 232), die aus einem Material mit negativer elektrischer Permittivität bei der Wellenlänge λ0 ausgeführt sind und auf eine Seite der Scheibe aufgebracht sind, wobei diese reflektierenden Elemente ein periodisches Gitter aus parallelen durchgehenden Schlitzen definieren, das einen Bandpassfilter um die Wellenlänge λ0 herum bildet, wobei die Periode dieses Gitters aus durchgehenden Schlitzen zwischen λ0/n12 und 2λ0/n12 beträgt, wobei die Vorrichtung jedoch auch umfasst:
    - einen zweiten optischen Filter (6; 122), der über dem ersten optischen Filter gelegen ist und der sich von einer zweiten unteren Grenzebene (P6inf) aus bis zu einer parallelen zweiten oberen Grenzebene (P6sup) erstreckt, wobei dieser zweite optische Filter eine Scheibe (22) umfasst, die identisch mit der Scheibe (12) des ersten optischen Filters ist, und reflektierende Elemente (24), die auf eine Seite dieser Scheibe des zweiten optischen Filters aufgebracht sind und die ein periodisches Gitter aus Schlitzen definieren, das identisch mit dem periodischen Gitter aus Schlitzen des ersten optischen Filters ist,
    - eine Zwischenschicht (8), die sich von der ersten oberen Ebene (P4sup) bis zu der zweiten unteren Ebene (P6inf) erstreckt, wobei diese Zwischenschicht einen Brechungsindex n8 bei der Wellenlänge λ0 und eine Dicke h8 aufweist, wobei der Brechungsindex n8 kleiner als 3n12/5 ist und die Dicke h8 zwischen δ und 3δ beträgt, wobei der Koeffizient δ durch die folgende Gleichung definiert wird: 1/δ = (2πn8/λ0) ((λ0/(n8P))2-1)0,5, wobei P die Periode des Gitters aus Schlitzen ist.
  2. Vorrichtung nach Anspruch 1, bei der die Dicke h8 zwischen 1,25δ und 2,75δ beträgt.
  3. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der die Breite der Schlitze größer als P/5 ist.
  4. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der die reflektierenden Elemente (14) ein einziges Gitter aus periodischen Schlitzen definieren und die Breite der Schlitze dieses periodischen Gitters größer als P/3 ist.
  5. Vorrichtung nach Anspruch 3 oder 4, bei der die Breite der Schlitze kleiner als 2P/3 ist.
  6. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der die reflektierenden Elemente (14; 232) des ersten Filters bündig mit der ersten oberen Grenzebene sind und die reflektierenden Elemente (24) des zweiten Filters bündig mit der zweiten unteren Grenzebene sind.
  7. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der die reflektierenden Elemente (14, 24) aus Silber oder aus Aluminium ausgeführt sind.
  8. Vorrichtung nach einem der vorhergehenden Ansprüche, bei welcher der Brechungsindex n8 kleiner als oder gleich 1,5 ist.
  9. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der:
    - die Zwischenschicht (8) aus einem elektro-optischen Material ausgeführt ist, dessen Brechungsindex n8 in Abhängigkeit von der Amplitude E des elektrischen Feldes, das die Zwischenschicht durchquert, variiert und
    - die Vorrichtung umfasst:
    - eine erste Anschlussklemme (156), an welche die reflektierenden Elemente (14) des ersten optischen Filters elektrisch angeschlossen sind, und
    - eine zweite Anschlussklemme (158), an welche die reflektierenden Elemente (24) des zweiten optischen Filters elektrisch angeschlossen sind, wobei die ersten und zweiten Anschlussklemme elektrisch voneinander isoliert sind.
  10. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der die Anzahl von Schlitzen des Gitters aus Schlitzen größer als λ0/Δλ ist, wobei Δλ die Breite auf halber Höhe des Transmissionspeaks bei der Wellenlänge λ0 ist.
  11. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der:
    - die orthogonale Projektion jedes reflektierenden Elements auf eine der Grenzebenen ein Parallelogramm ist, bei dem zwei Seiten parallel zu einer ersten Richtung sind und bei dem die beiden anderen Seiten parallel zu einer zweiten Richtung sind, die nicht kollinear zu der ersten Richtung ist,
    - diese reflektierenden Elemente periodisch zueinander in den ersten und zweiten Richtungen angeordnet sind, um ein erstes periodisches Gitter aus durchgehenden Schlitzen, die parallel zu der ersten Richtung sind, und ein zweites periodisches Gitter aus Schlitzen, die parallel zu der zweiten Richtung sind, zu definieren.
  12. Vorrichtung nach Anspruch 11, bei der die orthogonale Projektion jedes reflektierenden Elements auf eine Grenzebene ein Quadrat ist.
  13. Vorrichtung nach einem der vorhergehenden Ansprüche, bei der die reflektierenden Elemente aus Metall ausgeführt sind und ihre Dicke größer als die Eindringtiefe bei der Wellenlänge λ0 ist.
  14. Regelbares optisches Filtergerät, dadurch gekennzeichnet, dass dieses Gerät umfasst:
    - eine optische Filtervorrichtung (152) nach Anspruch 9,
    - eine Spannungsquelle (154), die elektrisch zwischen den Anschlussklemmen (156, 158) der Filtervorrichtung angeschlossen ist, wobei diese Spannungsquelle geeignet ist, eine Potentialdifferenz zwischen diesen beiden Anschlussklemmen zu erzeugen, wobei diese Spannungsquelle ein Modul (170) zum Regeln der erzeugten Potentialdifferenz umfasst.
  15. Detektor, umfassend:
    - eine oder mehrere Detektionszellen zur Detektion einer einfallenden elektromagnetischen Strahlung und
    - eine Vorrichtung (140a, 140b; 208) zum Filtern der einfallenden elektromagnetischen Strahlung, die vor jeder Detektionszelle angeordnet ist, dadurch gekennzeichnet, dass die Filtervorrichtung eine Filtervorrichtung nach einem der Ansprüche 1 bis 13 ist.
EP21191574.9A 2020-09-10 2021-08-16 Optische filtervorrichtung Active EP3968064B1 (de)

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